Thermionic amplifier



l Dec. 9, 1941. R. B. DOME v THERMIONIC v AMPLIFIER Filed May 29, 1940 Patented Dec. 9, 1941 THERMIONIC AMPLIFIER Robert B. Dome, Bridgeport, Conn., assigner to General Electric Company, a corporation of New York Application May 29, 1940, Serial No. 337,841

10 Claims.

My invention relates to a thermionic amplier and particularly to an amplitude limiting circuit for substantially removing amplitude modulation from high frequency oscillations translated therethrough While reproducing the frequency characteristics thereof with fidelity.

While it is to be understood that my invention is not limited thereto, it `will be apparent to 'those skilled in the art that lan amplifier pos- 4in the transmission circuits themselves or in the medium through which the Waves are propagated. Unless this amplitude modulation is removed, the received signal Will suffer distortion and the full advantages of phase or frequency modulation will not be realized, as is understood in the art.

It is therefore an object of my invention to provide an improved amplitude limiting circuit for substantially removing amplitude modulation from high frequency oscillations and which is particularly, though not necessarily, adapted for use in a phase or frequency modulation transmission system.

A further object of my invention is to pro-` vide an improved translating circuit for limiting the amplitude of translated oscillations to a substantially constant value irrespective of the amplitude of the envelope of the applied oscillations, while reproducing the frequency characteristics thereof faithfully.

Still another and more specific object of my invention is to utilize the inherent capabilities of a self-biased thermionic amplifier to provide a partially effective amplitude limiting action and to provide means for automatically varying the electrical characteristics thereof in accordance with amplitude fiuctuations in the average value of the self bias so as to eiiect a greatly improved limiting action.

The features of my invention which I believe invariably introduced. This may be to be novel are set forth With particularity in the appended claims. My invention itself, Vhowever, both as to its organization and method of operation, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawing, in which Fig. l diagrammatically represents one form of an amplitude limiting circuit embodying my invention, and Figs. 2 through 8, inclusive, represent curves and graphs helpful in analyzing and understanding the operating characteristics of the circuit of Fig. 1.

Referring now to Fig. 1, the electron discharge device I0 is a multi-electrode tube, illustrated as a pentode having a cathode II, conventionally represented as of the indirectly heated type, a control grid I2, a screen grid I3, a suppressor grid I4, and an anode I5. Oscillations to be translated by the device I0 are applied to an input circuitextending between control grid I2 and cathode II through any suitable coupling means, illustrated as the transformer I6. The secondary winding thereof is tuned substantially to the frequency of the applied oscillations by means of a capacitor I1. The input circuit also serially includes a resistor I8 and a capacitor I9 in shunt thereto. These constitute a grid leak and grid condenser combination for developing a negative self bias potential for the grid I2 in response to received oscillations. As will be explained shortly, in order for an amplitude limiting action to result, it is necessary to proportion the resistor I8 and capacitor I9 so that the negative potential developed thereacross through grid rectification of the applied oscillations at all times equals or exceeds the cutoff bias for the device IIJ above a predetermined minimum operating level. In other Words, the potential of grid I2 must be maintained sufficiently negative with respect to the cathode II so that current flows in the anode circuit only during alternate half cycles of the applied oscillations or fractions thereof.

The anode circuit for the device I0 serially includes a resonant tank circuit comprising the inductance 20 and capacitor 2|, a decoupling resistor 22 and a suitable source of anode potential, indicated by the positive and negative terminals 23 and 24. The negative terminal 24 of this source is returned to the grounded cathode I I in the usual manner and a relatively large bypass capacitor 25 is connected in shunt to the resistor 22 and the source 23, 24. The tank circuit 20, 2| is tuned substantially to the same frequency as the input tank circuit I6, I1. anode circuitl may be coupled in any suitable avn'urnber of assumptions will be made. for the time being that the resistor V2'! is short'- "stantia'lly Yconstant at all times. `that high frequency oscillations, for example, fas represented Yin Fig. 5, are impressed on input transformer IEE and that these are substantially sinusoidal in shape. Through the action of grid i leak I8? and condenser It peak detection of these The mannerto a utilization circuit, as for example through the coupling network 2B.

The screen grid I3 is connected to a suitable source of potential, as for example the positive terminal 23 of the anode potential source, 1 through a resistor 21. is connected between the screen grid I3 and the A by-pass capacitor` 28 cathode II. As will be explained below, the reaction obtained in accordance with myeinvention.

. Although the values of resistanceand capaci.- 1 tance of these respective elements are not sharp- 1 ly critical, it has been found by actual experiment that materially improved amplitude limit.-

I sistor 21 and the capacitor 28 play a very important part in the improved amplitude limiting ing action vin accordance with my invention is" ,Capacitor '2 8 is only large enough to provide a loyv reactance at the frequencies selected by vthe input and output tank circuits I6, Il and 2l.

Thus, the screen grid I3 is maintained substani tially at cathode potential for these frequencies, but its rpotential may var-y with respect to cath- 1 ode Hat lower frequencies. 'In general, capacitor` 2 8'must possess a relatively high reactance at vany frequencies which are present in the undesired ampli-tude modulation envelope and vwhich are to be eliminated, as will shortly become apparent.

,-l As previously mentioned, the oscillations, Whose envelope is to be limited to a substantially constant amplitude by the circuit of Fig. 1, may

f'be either of substantially constant frequency or i modulated in Yphase or frequency in accordance with signals to be conveyed, suchas audio frequency signals, for example. In the latter case the circuits I 5, I'I Vandr2, 2| should be broadly enough tuned to pass all frequencies Within the fdesired signal band with substantially equal'ef- Viicienc'y. It is to be understood that these carrier frequencies may be of any values substantially greater than the Vfrequencies which are present inthe undesired amplitude fiuctuations and which are to be eliminated. In general, the carrier frequencies will be radio frequencies well above the limits of audibility.

For an understanding of the operating characteristics'vof the circuit of Fig. 1, reference is now v mad-e to Figs. 2 through 8, inclusive, in conjunci tion with the following description( To simplify this analysis and to facilitate an explanation oftheY rather complex manner in which the circuit of Fig, `l is believed to function, Assume circuited'and that capacitor 28 is made large fenough to 'bypass all frequencies vdown to a very low value. The screen grid potential is then sub- Assume now tional-.pulses of .current will flow in the anode circuit during the positive half cycles of the applied oscillations, which reduce the instantaneous Agrid potential below the cutoff value. Two consecutive pulses are diagrammatically represented iby the solid line curve 30 in Fig. 2. This curve represents the variation in anode current alonga horizontal time axis. The 'horizontal portion of the curve 30 extending along the axis BE represents zero anode current, cor-responding to a value of vgridbias equal to cutoff. The crests 'Si of the anode current Wave are held to a maximum lanode current value corresponding substantially to zero grid bias potential. As is well known, this action is automatically secured by the grid -resistor IS and grid condenser I9, for

whenever the grid I2 becomes positive, additional self biasis instantly developed from the flow of grid current. This tbias in turn limits the anode current substantially to the zero bias Value.

It will be observed that .thus Vfar the operation of this circuit resembles that of the ordinary grid leak power detector, with the exception that the self bias developed by grid rectification is deliberately maintained at a Vhigh value at least equal to cutoff. The maximum height of the anode current pulses -is thus limited substantially vto the value AB, as'shown in Fig. 2. The crests 3| are limited to the voltage at whichY grid I2 becomes positive and the valleys are limited to the fixed horizontal line BE by conditions Aof anode current cutoff.

lIt will be apparent to those familiar with AFouriers analysis of complex wave forms that 'the non-sinusoidal w'ave 30 of Fig. 2 is analyzable into many component sine Waves, Vas well as a di.- rect current component. These sine waves comprise a fundamental wave of the frequency of the oscillations impressed on control grid I2 and many harmonics thereof. It will' be recalled that the output tank circuit 20,'2I is tuned substantially to this fundamental frequency. Hence it selects this frequency for transmission to the utilization circuit. The harmonic frequency components are bypassed by capacitor 2|.

Under the 'above assumed conditions, the input signal level is proportional to the maximum value AB of the Wave 30 in'Fig. 2. If this signal amplitude is selected as a reference level and arbitrarily designatedby unity, and if the anode current pulses are assumed to be portions of sine waves, then the maximum amplitude of the fundamental frequency component will theoretically equal .500, by Fouriers analysis. The direct current component will have a coefficient kof .318. f- Assume that the amplitude of the oscillation impressedV on the grid circuit is increased. This may result from amplitude variations in the received signal due to interference, such as noise, statc, or other factors. For convenience assume for the moment that the maximum amplitude of the waves is exactly doubled. The negative self bias applied to grid I2 will also be substantially doubled. Referring again to Fig. 2, this negative bias will establish a new axis CF for the anode current pulses corresponding to twice the cutoff value. The anode current wave may now be represented by the solid line curve 32. It is seen that anode current can still flow only during the interval AB since dotted line portions 32' of the curve in the interval BC are below the anode current cutoff line BE. This is fixed, since the potential of screen grid I3 remains constant under the assumed conditions. Thus, while the amplitude of the current wave, represented by the distance AB, is the same as before and limited by the same considerations, the anode current flows only during a fraction of positive half cycles. Therefore, these pulses 32 are narrower than the pulses 30 and include a smaller area. Fouriers analysis will immediately show that the coefficient of the fundamental component has been reduced to .387 and the direct current component to .220 due to the increase in input signal amplitude from 1.000 to 2.000, in terms of the arbitrarily selected reference level. Therefore, the fundamental component selected by the output tank circuit has been reduced in amplitude from .500 to .387 and effective limiting has not been accomplished.

The horizontal axis DG in Fig. 2 is representative of conditions where the grid bias is triple the cutoif value, since the distance AD is three times the distance AB. The input signal amplitude is thus 3.000 in terms of the reference level.

Under these conditions anode current ows during even a smaller fraction of positive half cycles and the current pulses 33 are still narrower since the dotted portions 33 below cutoff are twothirds the height of the curve. Fouriers analysis shows that the maximum amplitude of the fundamental frequency component is now further reduced to .330 and the direct current component to .175.

Thus, the above analysis indicates that the magnitude of the envelope of the oscillations selected by the output circuit 20, 2| steadily decreases as the amplitude of signal impressed on the grid I2 increases. This occurs despite the fact that the maximum amplitude of the anode current is substantially constant, since the area of the anode current pulses steadily decreases. The solid line curve 50 of Fig. 3 illustrates this graphically. The ordinates of this curve represent theoretical coefficients of the fundamental frequency component of anode current and the abscissae represent input signal amplitude, both in terms of the arbitrary reference level. The dashed line curve 5I illustrates the variation in the direct current component. I have confirmed the theoretical values of the curves and 5I by experimental tests and have found that they are in close correspondence with actual operating conditions.

The effect of amplitude variations in input signal level is further illustrated in Figs. 5 and 6 which represent oscillograms of signal potential and output current during conditions of a transient increase in signal amplitude, indicated by the positive peaks 40 in Fig. 5. It is seen that the envelope of the output current suffers a corresponding transient decrease, indicated by the negative peaks 4I. Thus the amplitude modulation of the received signal has not been eliminated by the circuit.

In accordance with my invention the mutual conductance of the device I0 is caused to vary automatically in accordance with the signal envelope. This I accomplish in the illustrated embodiment of my invention by causing the screen grid potential to vary inversely with the input signal amplitude through the action of resistor 2'! and capacitor 28, as will now be explained.

It is a well known characteristic of a pentode that when the control grid potential is varied, both the screen grid and anode currents vary in a similar manner. Thus, if the control grid I2 is made more negative, the current drawn by the screen grid through the series resistor 2'I decreases. The potential drop across resistor 21 likewise decreases and the screen grid potential increases, approaching the potential of supply source 23, 24 as the screen current approaches zero. Similarly, if the control grid I2 is driven more positive, the screen grid potential decreases.

As is well understood in the art, varying the screen grid potential varies the mutual conductance of the amplifying device in the same sense. Thus, if the screen grid I3 becomes more positive, the mutual conductance of device !0 is increased. Expressed another way, the anode to cathode resistance of device I0 is decreased and the control grid I2 must reach a more negative potential before anode current cutoff is reached. Thus the anode current cutoff axis BE in Fig. 2 is no longer fixed, Ibut moves downwardly with increased screen grid potentials. Considering the curves 30, 32 and 33 again, the eff-ect is to move the axis BE downwardly with increased input signal amplitudes so that the areas under the current pulses between the crests 3| and the axis BE are roughly constant in all cases. The curve 60 of Fig. 4 is illustrative of the general manner in which the mutual conductance of device I0 varies with the input signal amplitude as a result of the consequent Variations in screen grid potential.

As explained previously, capacitor 28 is of relatively small capacity. rlhis is necessary to permit the screen grid potential to follow the input signal envelope up to the highest envelope frequencies to be eliminated from the utilization circuit. As an illustration, if the circuit of Fig. l is employed as an amplitude limiter in a receiver of high frequency carrier waves which are frequency modulated by desired audio signals, the time constant of the resistor-capacitor combination 2l, 28 must be small enough to permit the screen grid potential to vary at audio frequencies. Thus, undesired fluctuations in the input signal due to noise or static interference at frequencies within the desired audio frequency signal band, as indicated by the peaks 40 in Fig. 5, for example, produce corresponding fluctuations in the screen grid potential, as illustrated by the curve 43 in Fig. 7. As a result, the negative peaks in the fundamental component of anode current, which appeared so prominently at 4I in Fig. 6 are substantially eliminated and the anode current is of substantially constant amplitude, as indicated in Fig. 8.

If perfect compensation were achieved, the curve 50 in Fig. 3, which represents the variation in the fundamental component of anode current without compensation., would assume the dotted line position 52. That is to say, the anode current would remain absolutely constant irrespective of the input signal envelope as long as its magnitude exceeded the minimum operating signal level. In general, perfect limiting 'cannot be attained, 'but it' can be very` closely approached.V The fundamental component of anode current Vcan be made to assume the position of the curve 53 in Fig. 3, for example. It will be observed that the Vlefthand portion is slightly under-compensated and the righthand portion slightly over-compensated, since curve 53 .is not linear. This non-linearity follows from the fact thatY the compensating action is the resultant of several independent non-linear variables which are not exactly identical functions of control grid potential. The screen grid current varies approximately as the three-halves power of control grid potential, as is well known, and the anode current Vvaries according to a slightly different law. f

By increasing the value of resistor 21 beyond the optimum value, the circuit may be considerably over-compensated, as indicated by the curve 53 in Fig. 3.

It has been found, as a practical matter, that the effectiveness ofV limiting varies considerably with the nature of the amplitude fluctuations to be eliminated. It is greater for slower variations than for sharply peaked noise transients, such as those due to ignition interference. However, even under the most adverse conditions, it is reasonable to expect an increased effectiveness of limiting of at least two-fold when the mutual conductance of device lil is caused to vary in accordance with the principles of my invention, as hereinbefore explained. The improvement in many cases is very much greater. Thus, in laboratory tests, when the input signal was amplitude modulated 50 per centy by a 400 cycle standard sine wave, an improvement of Ll-fold in effectiveness of limiting was obtained when capacitor 28 was reduced to l5 mmf. from 0.1 mfd.

It will be evident that my invention provides an amplitude limiting circuit which is structurally very simple and gives Van effective limiting action Well Within practical tolerances. By way of illustration only, and not by way of limitation, there are listed below values of circuit constants which have been found to be suitable for the circuit of Fig. 1 when employed in a particular superheterodyne frequency modulation receiver. The device I0 was a type 6SJ7 pentode and the circuits I6, I1 and 20, 2| were tuned to the intermediate frequency of about 2.1 megacycles. Other values follow:

Resistor 18=330,000 ohms Resistor 22:2,200 ohms Resistor 27=l00,000 to 300,000 ohms Capacitor 19:22 mmf.

Capacitor 28:15 mmf.

Capacitor :.05 mfd.

The optimum value of resistor 2'! was determined =by actual tests, as explained. It varied somewhat with the type of interference to be eliminated, though any values within the above range were found to be effective. v

It is to be understood that the foregoing analysis is merely illustrative of the fundamental manner in which the circuit of Fig. 1 is believed to operate. Various assumptions as to wave shapes and operating conditions have been made for the purpose of simplifying this explanation. It will be appreciated that these are not essential to the practice of my invention but are merely exemplary. The electrical behavior of the circuit under actual operating conditions isundoubtedly more complex since, as previously pointed out,

have demonstrated the effectiveness and practical utility of my invention.

IAlthough I have illustrated one embodiment of my invention employing a pentode, the mutual conductance of which is controlled by variation of the screen grid potential, other modifications will readily occur to those skilled in the art by which the same results may be achieved. For example, the device 1 0 may be a screen grid tube rather than pentode, or the device I0 maybe a multi-grid tube having the compensating potential applied to some other grid than the screen grid. Therefore, it will be appreciated Vthat I do not Wish to be limited to fthe particular embodiment of vmy invention disclosed herein since these and other modifications may be made, and I contemplate by the appended-claims to cover any such modifications as fall Within the true spirit and scope of my invention. l

What I claim as new and desire to secure by Letters Patent of the United States is:

1. The combination, in an amplitude limiting circuit, of an electron discharge device having grid and anode circuits, means for impressing high frequencyl oscillations on said grid circuit, means for developing in said grid circuit an average self-bias potential sufiicient to maintain said device biased at least to cutoff above a predetermined minimum amplitude of said oscillations at which the limiting action becomes effective, means in said output circuit to select oscillations of the same high frequencies, and means responsive to changes in said bias potential for increasing the mutual conductance of Vsaid device when said potential becomes more negative, and vice versa. Y

2. In a circuit for amplifying received high frequency oscillations and for limiting the amplitude thereof, the combination of a thermionic translatins device having a cathode, a plurality of control electrodes and an anode, an input circuit between said cathode and one of said control electrodes, means to impress said oscillations on said input circuit, means to develop a bias in said input circuit in response to said oscillations sufficiently great to maintain conditions of anode current cutoff for all amplitudes of said oscillations above a predetermined operating level at which the limiting action becomes effective, an output circuit between said anode and cathode including means for selecting currents of the frequencies of said oscillations, whereby undesired amplitude variations in said oscillations tend to produce amplitude variations in said output currents, and means responsive to variations in said bias for varying the potential of another one of said control electrodes inversely therewith, whereby said current variations are substantially reduced.

3. In combination, a thermionic amplifier having grid and anode circuits, means to impress oscillations on said grid circuit, means to produce in said grid circuit a bias potential substantially proportional to the amplitude of said oscillations, said potential being at least sumcient to maintain said amplifier at cutoff above a predetermined minimum operating level, output means in said anode circuit to select currents of said high frequencies Whose amplitude tends to vary in accordance with the amplitude of said oscillations, and means to vary the mutual conductance of said amplifier in response to variations in said bias potential and in a sense to reduce said current variations.

4. In an amplitude limiting circuit, the combination, with a source of oscillations whose envelope is subject to undesired amplitude variations, of a thermionic discharge device having grid and anode cir-cuits, means to impress said oscillations on said grid circuit, means to bias said grid in accordance with the envelope of said oscillations, said bias being sufiicient to maintain anode current cutoff above a predetermined minimum operating level at which the limiting action becomes effective, means in said anode circuit to select corresponding oscillations, and means responsive to said undesired variations in the envelope of said first-named oscillations for increasing the mutual conductance of said device when said envelope amplitude increases, and vice versa.

5. In an amplitude limiting circuit, the combination, with a source of oscillatory waves whose envelope is subject to amplitude variations at undesired frequencies, of a thermionic amplifying device comprising a cathode, a control grid, a screen grid and an anode, an input circuit between said control grid and cathode, means for coupling said source to said input circuit, means for biasing said control grid to cutoff in response to said Waves comprising a grid leak and grid condenser in said grid cir-cuit, an anode circuit between said anode and cathode tuned to select oscillatory currents corresponding to said waves, a circuit serially including a resistor and a source of positive potential for said screen grid connected between said screen grid and cathode, and a capacitor connected between said screen grid and cathode which has a relatively low reactance at the frequency of said waves and a relatively high reactance at said undesired frequencies.

6. In an amplitude limiting circuit, the combination, with a source of oscillatory waves whose envelope is subject to amplitude variations at undesired frequencies, of a thermionic amplifying device comprising a cathode, a control grid, a screen grid and an anode, an input circuit between said control grid and cathode, means for coupling said source to said input circuit, means for biasing said control grid to cutoff in response to said waves comprising a grid leak and grid condenser in said grid circuit, an anode circuit between said anode and cathode tuned to select oscillatory currents corresponding to said Waves, a circuit serially including a resistor and a source of positive potential for said screen grid connected between said screen grid and cathode, and a capacitor connected between said screen grid and cathode, the impedance of said resistor and capacitor in parallel being relatively great at the frequency of said waves and relatively small at said undesired frequencies, whereby the instantaneous potential of said screen grid is caused to vary at said undesired frequencies.

7. In an amplitude limiting circuit, the combination, with a source of oscillatory waves whose envelope is subject to undesired amplitude modulation, of an electron discharge device having a cathode, a plurality of grids and an anode, an input circuit between said cathode and a first one of said grids, means for coupling said source to said circuit, means for biasing said rst grid to cutoff in response to said waves comprising a grid leak and grid condens-er in said circuit, anv output circuit between said anode and cathode in which pulses of anode current flow duringr portions of positive half cycles of said oscillatory Waves, whereby the envelope of the fundamental frequency component of said anode current pulses and the direct current component thereof tend to vary with said undesired amplitude modulation, and means responsive to `said undesired envelope variations for increasing the potential of a second of said grids when said envelope amplitude increases, and vice versa, whereby said variations in said components are substantially reduced.

8. A limiter circuit for amplifying and limiting high frequency waves whose envelope is subject to undesired amplitude modulation at lower frequencies comprising, in combination, a pentode amplifier, an input circuit for coupling said oscillations between the control grid and cathode of said amplifier, a parallel resistance-capacitance combination in said input circuit for effecting grid detection of said oscillations and developing a self bias potential for said grid, said combination being proportioned to maintain said amplifier at cutoff above a predetermined minimum amplitude of said waves, an output circuit between the anode and cathode of said amplifier serially including an output means for selecting waves of said high frequencies and a source of anode potential, a circuit between the screen grid and cathode of said amplifier serially including a resistor and a source of screen grid potential, and a bypass capacitor connected between said screen grid and cathode which has a low reactance for said high frequencies and a high reactance for said undesired frequencies.

9. A limiter circuit for amplifying and limiting high frequency waves whose envelope is subject to undesired amplitude modulation at lower frequencies comprising, in combination, a pentode amplifier, an input circuit for coupling said oscillations between the control grid and cathode of said amplifier, a parallel resistance-capacitance combination in said input circuit for effecting grid detection of said oscillations and developing a self bias potential for said grid, said combination being proportioned to maintain said amplifier at cutoff above a predetermined minimum amplitude of said waves, an output circuit between the anode and cathode of said amplifier serially including an output means for selecting waves of said high frequencies and a source of anode potential, a circuit between the screen grid and cathode of said amplifier serially including a resistor and a source of screen grid potential, and a bypass capacitor connected between said screen grid and cathode, the impedance of said resistor and capacitor in parallel being relatively great at the frequency of said Waves and relatively small at said undesired frequencies, whereby the instantaneous potential of said screen grid is caused to vary at said undesired frequencies.

l0. An amplitude limiter comprising, in comprising, in combination, a multi-grid amplifier including a cathode, a control grid, a screen grid and an anode, an input circuit between said cathode control .grid including means to impress high frequency oscillations thereon whose envelope is subject to undesired variations in amplitude and also including self bias means to maintain said amplifier biased at least to cutoff above a predetermined minimum amplitude of said oscillations, a circuit between said anode and cathode including frequency selective output means to select` high frequency currents correspendingtosaid oscillations,Y whereby said currents-are; ais'o subject to undesired Variations, and means to cause the potential of said-screen grid-to Varywith respect to said cathode in accordance WithY frequencies present in said undesire'dfamplitude variations, whereby said Variations are substantially reduced, said?.y last nar'ned means comprising a resistor includedv in the connection to said screenr 'grid and a'luypass capacitor connected between said screen; grid and cath-V ode having a low reactance for'o'scill'aton frequencies and a high reactancefor frequencies present in said undesiredamplitude variations.

` A ROBERT B. DOME. 

